0 :: nat
bool :: nat → boolean
cons :: nat → list → list
consif :: boolean → nat → list → list
false :: boolean
filter :: (nat → boolean) → list → list
nil :: list
rand :: nat → nat
s :: nat → nat
true :: boolean
rand(X) → X
rand(s(X)) → rand(X)
bool(0) → false
bool(s(0)) → true
filter(F, nil) → nil
filter(F, cons(H, T)) → consif(F(H), H, filter(F, T))
consif(true, H, T) → cons(H, T)
consif(false, H, T) → T